Lasers are used in a variety of applications (e.g., in optical networks), and the operation of lasers can be controlled by modulating the current of a laser using a laser driver. Lasers use particular bias currents to set an appropriate operating point for the laser. For example, the bias current can be used to set the optical power to represent a logical ‘0’ level in a signal used for laser transmissions (the “bias point”). A modulation current can be used to set the optical power of a logic ‘1’ level of a signal used for laser transmissions (the “modulation point”), where the logic ‘1’ level is set relative to the ‘0’ level that is determined by the bias current. In many applications (e.g., burst mode and continuous mode), different control schemes are applicable for setting the bias current and modulation current according to temperature, monitor photo diode (MPD) average levels, etc.
For example, a laser may be used to transmit different levels of an optical signal representing binary data. If there is a small difference between the ‘0’ optical level and the ‘1’ optical level, it can be difficult for a receiver to differentiate between the two levels and extract the content of the transmitted data. The two transmitted levels are directly correlated to laser characteristics, such as the Current to Optical characteristics (how much current is converted to how much light), and so the two transmitted levels are directly related to the currents that the laser driver drives through the laser.
Thus, a laser driver ideally tries to maintain an average configured optical power while trying to maintain a large enough difference between the ‘0’ optical level and the ‘1’ optical level. For a burst mode transmission, where the modulated signal is directly added to a bias current signal, the ‘0’ optical level is generated by the bias signal, while the ‘1’ optical level is generated by the bias current+the modulation current. Thus, the laser driver should ideally attempt to generate a current that will transmit an optical signal of ‘0’ as close as possible to the target optical bias point and transmit a ‘1’ as close as possible to the target modulation point so that a receiver can easily differentiate between a transmitted ‘0’ and ‘1.’ Typically, most systems only monitor the average power (the average of the power transmitted between the bias point and the modulation point) because of the difficulty in extracting the actual high speed optical levels of transmitted ‘0's’ and ‘1's.’
A key challenge in designing a laser system is generating a stable, accurate bias current and modulation current that ensure the required optical characteristics. This challenge is complicated by the laser's changing of the Current to Optical characteristic as temperature and the lifetime of the laser varies. Achieving the correct levels of bias and modulation is important for improving optical signal quality, including improving optical signal quality factors such as jitter, extinction ratio, duty cycle distortion, etc. Embodiments of the present disclosure provide methods and systems for efficient, accurate bias and modulation current control of a laser driver.
Features and advantages of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.